1. Field of the Invention
The present invention relates generally to an apparatus and method for generating short pulses of laser energy and, more particularly, to passively modelocked lasers for providing short pulses with enhanced power.
2. Description of the Related Art
Both actively and passively modelocked lasers are useful as short pulse sources, and frequently employ single-mode rare-earth doped fibers as the lasing material. To be reliable and therefore commercially practical, a fiber-based short pulse source must be environmentally stable, i.e. it should be substantially immune to environmental influences such as temperature drift and variations in pressure. Unfortunately, conventional fiber-based short pulse sources are susceptible to both temperature and pressure variations and require constant monitoring to maintain pulse generation.
The simplest methods for generating short laser pulses involve passive modelocking. (See, for example, U.S. Pat. No. 5,689,519 to Fermann et al. and U.S. Pat. No. 5,448,579 to Chang et al.) The three most common passive modelocking techniques involve either a saturable absorber inside the cavity, nonlinear polarization evolution, or a combination of both nonlinear polarization evolution and a saturable absorber. As discussed by Fermann et al., the exploitation of nonlinear polarization also favors the production of the shortest possible pulses. Environmentally-stable cavities that comprise a saturable absorber are most conveniently constructed in a Fabry-Perot cavity.
Passive modelocking techniques based upon saturable absorbers are the most promising and permit the construction of relatively simple and reliable cavities. Fiber lasers that use saturable absorbers have been described by Loh et al. ("All-solid-state subpicosecond passively mode locked erbium-doped fiber laser", Appl. Phys. Lett., vol. 63, pp. 4-6, 1993), Barnett et al. ("High-power erbium-doped fiber laser mode locked by a semiconductor saturable material") and Reddy et al. ("A turnkey 1.5 micron picosecond Er/Yb laser"). However, these designs suffer from polarization instabilities.
Another shortcoming with current modelocked designs is the inefficiency with which power is coupled out of the cavity. For example Barnett et al., Reddy et al., and Loh et al. describe systems in which the 4% reflecting end of an uncoated fiber is used as the output coupler. While this simplifies the cavity design, it does not optimize the laser performance with respect to stability and output power. Further, in Tamura et al. (U.S. Pat. No. 5,513,194), a design for optimizing the output power is described, but this design is not environmentally stable and does not include a saturable absorber.
In general, one shortcoming in the prior art is that output power is not optimized with respect to the pump power. However, to minimize the cost of a passively modelocked fiber laser, the output power of the fiber laser with respect to available pump power should be maximized, since the pump laser is often the most expensive component in a fiber laser system.
Thus, there remains a need for a mode-locked laser device that is environmentally stable, free of polarization instabilities, and energy efficient, thus giving enhanced power.